1. Field of the Invention
The present invention relates to a subscriber line interface circuit for a telephone terminal station having a digital switching system. Especially it relates to a battery feed circuit, a hybrid circuit and a supervision circuit for fabrication in monolithic ICs (integrated circuits) to miniaturize their size and to reduce their fabrication cost.
2. Description of Related Art
A subscriber line interface circuit for a telephone station is generally composed of seven fundamental circuits as shown in FIG. 1. Namely, a battery feed circuit for providing DC power for subscriber lines, a hybrid circuit for converting the telephone signal from 2-wire signal to 4-wire signal or vice versa, overvoltage protection circuit, ringing switch circuit for handling a ringing signal, CODEC (coding and decoding circuit) for telephone code, supervision circuit for supervising the operation of the circuit and test access relay circuit. All of them are provided respectively for each subscriber. Therefore, it is very important to reduce their size, power consumption and cost.
First, the CODEC has been fabricated in monolithic IC and is now in wide use, because the voltage applied to it is low (about 5 volts) and it was easy to realize in an IC. Attempts to fabricate the remaining circuits in an IC are beginning to be applied in practice. These efforts are directed to design ICs for the battery feed circuits, the 2- to 4-wire converter and the supervision circuit. The reason for selecting these circuits is that they deal with a medium voltage of 40-60 volts, while the remaining circuits deal with several hundreds volts or high current to be handled by relays.
The subscriber line interface circuit can be considered as equivalent to a circuit as shown in FIG. 2(a). To the terminals A and B are connected subscriber equipment having an impedance Z.sub.0 which includes the impedances of the subscriber line and telephone equipment, for example. The subscriber line interface circuit (abbreviated as SLIC hereinafter) is provided with an AC termination impedance Z that terminates the subscriber line, and a battery feed circuit that supplies a DC voltage to one subscriber line, A for example, and the other subscriber line B is grounded through a feeding resistance. A telephone signal from the subscriber is taken out from the terminals A and B via a hybrid circuit converting a 2-wire signal to a 4-wire signal. Therefore, the internal impedance of the battery feed circuit must be as high as possible for the telephone signal which appears in a differential mode having a phase difference between the lines A and B. But in order to prevent noise generated in the subscriber line, the internal impedance for the noise should be as small as possible. Noise is usually generated on the subscriber line in a longitudinal mode that has the same phase in the lines A and B. So, the internal impedance of the battery feed circuit is preferably as low as possible for the longitudinal mode signal, but it should be as high as possible for differential mode signal.
To realize such internal impedance, various circuits have been proposed. But they can be reduced to an equivalent circuit as shown in FIG. 2(b) for the longitudinal noise voltage. The equivalent circuit is a bridge circuit having two resistances R corresponding to the lines A and B respectively, and internal resistance R.sub.A and R.sub.B of battery sources for lines A and B respectively. The noise voltage Vn is applied to a junction point of the two resistances R in equal phase for both of the lines. Therefore, two main approaches have appeared in the design of SLICs to eliminate the noise voltage between the terms A and B. One approach is balancing the bridge circuit, that is to make EQU R.sub.A /R=R.sub.B /R,
the other is to make R.sub.A and R.sub.B as small as possible compared to R. The former is called a balanced type, and the latter is called a non-balanced type SLIC.
"A Monolithic Telephone Subscriber Loop Interface Circuit" by W. David Pace, IEEE Journal of Solid State Circuit, Aug. 1981, discloses fundamentals of monolithic IC circuits for the non-balanced type SLIC.
The following Japanese Laid Open Patents: Tokkai 55-150658; 56-141655; 57-25766; 57-38053; 58-104559 disclose balanced type battery feed circuits; Tokkai 58-210775; 59-161172 disclose non-balanced type battery feed circuits; and Tokkai 57-42263 discloses an IC circuit for the 2-to 4-wire converter.
The non-balanced type battery feed circuit has been more easily realized in a monolithic IC. It is important to make an equivalent value of the resistances R.sub.A and R.sub.B as small as possible. Using mirror circuits and feedback circuits, the equivalent value of R.sub.A and R.sub.B have been reduced to less than 1 ohm, and a longitudinal balance of 45 dB has been attained, when the circuit is trimmed. But the larger the longitudinal balance the better.
In order to attain higher longitudinal balance, it is necessary to balance the bridge circuit of the equivalent circuit of FIG. 2(b). This is the balanced type circuit. But in prior art circuits, it was necessary to use discrete components such as capacitors, power transistors and so on, which are difficult to fabricate in a monolithic IC. Moreover, the balance was liable to be disturbed by the variation of subscriber line impedance, supply voltage, signal level and so on, and when the balance was disturbed the circuit suffered from noise, so a precise adjustment of the balance is necessary.